Nuclear transplantation: reprogramming of transplanted nuclei.

Recent results in animal cloning have demonstrated that the events of cell differentiation can be reversed. This reversal necessarily requires genetic reprogramming of the transplanted nucleus. It has emerged from review articles that synthesis of nuclear and ribosomal RNA is blocked after fusion independently of cell cycle stage of the transferred nucleus, and that the cytoplasm condition of the cytoplast controls the speed of this inhibition. Transcription of the embryonic genome in nuclear transplant embryos begins either at the same stage as in normal development or even earlier. Expression analysis of individual genes showed up differences between nuclear transplant and normal embryos at the blastocyst stage. A certain degree of nuclear reprogramming after fusion is necessary, but exact timing and the qualitative sequence of the processes are probably not necessary for the successful embryonic development of the cloned embryo. In the future, observation of the expression of individual genes will bring more exact information about nucleus reprogramming.